Wheel-hub bearing units of the known type and in their simplest configuration comprise:                an outer bearing ring which is stationary when in use and connected to a pillar of a vehicle;        a pair of radially internal bearing rings which are not stationary when in use and are mounted axially alongside each other on a flanged wheel hub which can be connected when in use to a wheel of the vehicle; and        two rings of rolling members interposed between the outer ring and the pair of radially internal bearing rings so as to rotatably and operatively connect the flanged wheel hub to the pillar of the vehicle.        
The wheel-hub bearing units described above are of the first generation type, where the outer bearing ring is connected to the pillar by means of insertion of the outer ring itself inside a cylindrical housing seat created in the pillar itself and is axially locked inside the cylindrical housing seat either by means of cold-deformation of one or both the axial opposite ends of the outer ring adjacent to the pillar or by means of use of additional mechanical locking elements such as, for example Seeger rings, which must be coupled with the inner ring and then inserted, together with the inner ring, inside the cylindrical housing seat for locking together thereof.
Wheel-hub bearing units of the known type and in what is considered to be their more complex configuration comprise instead:                an outer bearing ring which is stationary when in use and has an outer flange for connection thereof to a pillar of a vehicle;        a central flanged hub and a bearing ring which are radially internal and not stationary when in use, the central flanged hub thereof being able to be connected, when in use, to a wheel of the vehicle, and the radially internal bearing ring being mounted integral with the central hub; and        two rings of rolling members interposed between the outer ring and, respectively, between the central hub, on which a raceway for rolling members of the associated ring is directly formed, and the radially internal bearing ring, on which a further raceway for rolling members of the associated ring is formed.        
The wheel-hub bearing units just described are, instead, of the third generation type and, if they offer undoubted technical advantages compared to the wheel-hub bearing units described above, i.e. those of the first generation type (advantages such as to be preferred by the OEM in the so-called “original equipment sector”, i.e. during assembly on production vehicles), they do not appear to be sufficiently flexible and low-cost in terms of their use for the “aftermarket”, i.e. in the so-called Vehicle Service Market.
In fact, if in “original equipment sector” the wheel-hub bearing units must have technical characteristics which take into account the whole life cycle of the vehicle on which they are mounted, in so-called “aftermarket”, the wheel-hub bearing units must have technical characteristics which take into account only the remaining life cycle of the vehicle. Therefore, any simple replacement of a wheel-hub bearing unit of the third generation type with a similar wheel-hub bearing unit is not convenient, not only for the user of the vehicle, who would have to incur costs higher than those strictly necessary, but likewise for the spare parts manufacturer, who would also have to continue the production of the wheel-hub bearing units for vehicles which may not even be on the market any longer.